Riesgo said: 'Understanding when sea sponges emerged and when they diverged from other animals is vitally important, because it helps us to understand the evolution of all life on Earth.'

Traces of early life

Riesgo and her colleagues studied the genetic toolkits that produced the fossilised molecules between 650 and 540 million years ago.

A fossilised sea sponge from Utah dating from the Cambrian period, which began about 542 million years ago

Molecular fossils are organic molecules from dead organisms that have been preserved.

They are particularly important in the study of very early life forms because traditional fossils - preserved remains or impressions made by an animal or plant - do not exist.

The new paper proposes that a sterol, a chemical called 24-isopropylcholestane (24-ipc) which is found in the fossils, was produced by sea sponges, the simplest multi-cellular animals on the planet.

24-ipc is also produced by some types of algae and several recent scientific papers have proposed that ancient algae are the source of the chemical.

But in their paper, Riesgo and her colleagues explain that algae are unlikely to be the source in rocks that are so old, because these organisms did not possess the necessary gene to produce 24-ipc until significantly later.

Reading the molecular clock

The team carried out genetic analysis of the four major groups that evolved from ancient sea sponges and their closest relatives, and identified the gene responsible for 24-ipc production in the most abundant group of sponges.

They were then able to calculate when this gene evolved in sponges and algae using a molecular clock, which tracks the mutation rates of the genes and calculates when organisms have diverged.

The results suggested that algae and sponges developed the ability to produce 24-ipc independently of each other, but sponges were producing the chemical much earlier.

Riesgo said: 'Our study provides strong evidence of sponges being responsible for the early traces of 24-isopropylcholestanes, not algae. It suggests that sponges predated the Cambrian explosion, despite the scant fossil record of sponges in that period.

'Our results reinforce the usefulness of molecular fossils, and particularly sterols, for understanding the evolution of animals.

'In these times of uncertainty around the specific evolutionary relationships of the earliest animals, biomarkers and molecular-clock approaches with biologically relevant enzymes can help unravel the evolutionary pathways that resulted in the present diversity of animals.'

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